This invention relates to microwave ovens and, more particularly, to the ovens that have materials continuously passing through them. For such continuous processes a means of access must be provided to the oven in order to permit the passage of a conveyor belt and/or the product. However, any physical opening or access port in a microwave oven creates some leakage hazard to the operating personnel. Moreover, substantial leakage occurs for any practical sized opening and the amount of energy released is even more hazardous because of the typical operating frequencies of the microwave ovens.
Previous structures have utilized various types of filters, cavities, cutoff structures, capacitive structures, slot reflectors, and water jackets. In particular, a water jacket forming an annulus around the conveyor belt and product has the advantages of conveniently carrying off the leakage energy heat and having a high microwave loss. Typical water jacket structures that have been proposed contain the water between a metal outer jacket and an inner insulation or tubing. The outer jacket serves both as a reflective member and also a containment for the microwave energy. The inner insulation comprises the actual envelope through which the conveyor belt longitudinally passes and through which the microwave energy transversely passes into the water.
The use of an annular water jacket has the difficulty of reflecting an excessive portion of the microwave energy back into the open area around the conveyor belt and not efficiently absorbing the microwave energy. The dielectric constant of water at microwave frequencies is about 80, whereas the dielectric constant of air is approximately 1. The discontinuity between these two dielectric constants is so great that only about one-seventh of the total energy impinging upon the water surface is able to actually pass into it. Thus, even though water is one of the best microwave attenuation mediums, the amount of reflection at an air-water boundary requires that known end loads be made quite long in order to be sufficiently effective.
In addition, the prior structures, such as that shown in U.S. Pat. No. 3,754,111, entitled "Access Tunnel and Attenuator for Microwave Ovens", issued to Peter D. Jurgensen and assigned to the same assignee as the present application, have required dimensional tolerances that are difficult to obtain by normal manufacturing methods. There is, therefore, a need for an improved microwave oven end load.